LT3464

6

3464f

The LT3464 uses a constant off-time control scheme in

conjunction with Burst Mode

efficiency over a wide range of output current. Operation

can best be understood by studying the Block Diagram.

When the FB pin voltage is lower than the 1.25V reference,

the hysteretic comparator enables the power section,

causing the chip to start switching, thus charging the

output capacitor. When the output voltage increases

enough to overcome the hysteresis, the feedback com-

parator shuts off the power section leaving only low power

circuitry running until the output voltage falls again. This

cycle repeats, keeping the output voltage within a small

window. The switching action is as follows: The switch

turns on, and current through it starts to ramp up until the

OPERATIO

point where the current limit is reached, at which point the

switch turns off for a fixed amount of time. While the

switch is off the inductor is delivering current to the load.

When the off time expires, the switch turns on again until

the current limit is reached, and the cycle repeats.

This chip includes an internal power Schottky diode and a

PNP transistor for output disconnect. The PNP transistor

disconnects the load from the input during shutdown. The

PNP control circuitry is designed to keep the PNP out of

saturation across a wide range of current, to keep quies-

cent current to a minimum and to provide current limiting

to protect the chip during short-circuit conditions.

Burst Mode is a registered a trademark of Linear Technolgy Corporation.

BLOCK DIAGRA

S

R

Q

Q

DELAY

–

–

+

–

+

12mV

OUT

OUT

1.25V

CTRL

SW

GND

OUT

ANTI

SAT

4

8

1

2

7

6

5

3

SHDN

FB

CAP

3464 BD

0.1

Ω